Graphene is a 2-dimensional structure of carbon atoms arranged in a hexagonal crystal lattice. Graphene has high electrical conductivity, high thermal conductivity, and high mechanical stiffness despite its small thickness (˜3.4 Å). Due to such characteristics, graphene has been in the spotlight as a material for semiconductor devices that has the potential to replace silicon in the near future. The high electrical conductivity and high mechanical stiffness of graphene make it easier to produce flexible substrates. Based on these physical properties, graphene has attracted attention as a transparent electrode material capable of replacing indium tin oxide (ITO).
Graphene oxide is stable due to its high solubility. Accordingly, graphene is stored and transported in its oxide form and graphene oxide is reduced for use where conductive graphene is required. However, graphene oxide is not completely reduced. In some cases, reduced graphene oxide is again oxidized. Test methods for graphene are thus considered important.
According to conventional methods for testing mass-produced large-area graphene, the presence or absence of defects in graphene is determined by observing a change in temperature distribution after a current is applied to the graphene. Large-area graphene loses its conductivity when partially oxidized. In this case, the application of current causes a difference in electrical resistance between oxidized and reduced regions of the graphene. The different resistance values lead to a difference in the amount of heat generation upon the application of current, and as a result, the thermal distribution of defective regions (oxidized regions) is distinguished from that of defect-free regions (reduced regions). By inspecting the different thermal distributions using a thermal imaging camera, a determination can be made as to whether the graphene is defective or not.
However, when the defective regions are monitored through their thermal distributions, it is impossible to determine the exact position and size of the defective regions. No report has appeared on more precise apparatuses and methods for testing mass-produced graphene to determine the position and size of defective regions in the graphene.
In this connection, Korean Patent Publication 10-2013-0114617 discloses a method for testing graphene substrates using ultraviolet light. However, this method requires a darkroom to use ultraviolet light, causing inconvenience for users, and utilizes the ultraviolet light transmittance of graphene through a difference in the formation of graphene layers rather than the electrical conductivity of graphene itself, limiting its accuracy.